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By Richard Cowan

WASHINGTON (Reuters) - The National Rifle Association is on track for record spending this year on U.S. political campaigns, including Missouri's U.S. Senate race where a gun-owning Democrat is waging a tough fight against an incumbent Republican backed by the gun rights group.

Democrat Jason Kander, an Afghanistan war veteran, is running just 2.5 percentage points behind Republican U.S. Senator Roy Blunt in opinion polls in Missouri, despite the NRA's expenditure of $1.8 million so far trying to protect Blunt as well as the Republicans' U.S. Senate majority.

Gun violence has transfixed the United States in 2016, from a massacre at an Orlando, Florida, nightclub to a rash of police shootings, fueling demands by some Democrats and activists for tighter gun laws.

Those efforts have been thwarted, as in years past, by the NRA and its supporters, who say such measures would infringe on the right to bear arms guaranteed by the Second Amendment of the U.S. Constitution. Congress has not approved major gun-control legislation since the 1990s.

An Aug. 19 Reuters/Ipsos poll found 41.8 percent of respondents agreed there should be strong restrictions on firearms and 22.4 percent supported moderate regulations. Per-capita firearm ownership in the United States far exceeds any other country, according to government estimates.

In the Missouri race and other Senate contests in states such as North Carolina, Nevada, Florida and New Hampshire, the gun-control battle gripping Washington is playing out on the campaign trail.

The NRA has spent $23.4 million in this election cycle, with a month to go before voting on Nov. 8, compared with a previous high of $27 million in the entire 2014 campaign cycle, according to federal data gathered by the Center for Responsive Politics.

The NRA did not respond to requests for comment.

In addition to Senate Republicans, the lobbying group is also backing Republican presidential nominee Donald Trump.

Blunt, 66, is a two-decade veteran of Congress. He accuses Kander, 35, of being soft on gun ownership rights.

Blunt has an "A" rating from the NRA. Kander has an "F," which the NRA says he earned by backing expanded background checks on gun buyers and legislation that failed this year in Congress to bar people on "terrorism watch lists" from buying guns.

"The difference between Senator Blunt and myself is that I want to stop criminals and suspected terrorists from having the same access to guns as the rest of us, which is why I support background checks," Kander said in a statement to Reuters on Thursday.

'RESET THE DEBATE'

Kander, Missouri's secretary of state, counterpunched against Blunt last month with a 30-second TV commercial that has had more than 1 million views on youtube.com.

In the ad, the former U.S. Army captain assembles a military-style rifle while blindfolded. He says he has supported gun ownership rights, but adds: "I also believe in background checks so that terrorists can't get their hands on one of these."

A fierce debate in Washington over such background checks in June led to a 25-hour sit-in by Democrats on the floor of the House of Representatives.

"The reaction to that ad has reset the debate" in the Missouri Senate race, said Tim Daly, managing director of guns and crime policy for the liberal Center For American Progress, a think tank in Washington.

Kander's polling numbers were already rising when the Sept. 15 ad was first broadcast. But Daly said Kander planted fresh doubts over the NRA position on background checks.

Blunt and his campaign spokesmen were not available for comment.

The NRA is Blunt's No. 2 financial backer in the Missouri race, surpassed only by a group closely associated with U.S. Senate Majority Leader Mitch McConnell that backs Republicans.

SENATE MAJORITY AT STAKE

Like the Blunt-Kander battle, other key 2016 Senate races feature intense debate over guns as Republicans work to defend their control of the Senate.

In North Carolina, the NRA has pumped in $2.8 million to support threatened incumbent Republican Senator Richard Burr.

The first televised debate in New Hampshire this week between Republican Senator Kelly Ayotte and her Democratic challenger, Governor Maggie Hassan, began with a moderator asking how they could reduce gun violence without infringing on gun rights.

Ayotte has come under attack from Americans for Responsible Solutions for her opposition to tough gun background checks. The group works to reduce U.S. gun violence in the United States.